Calculating gas using usize (if supplied gaslimit fits in usize) (#1518)

* Spliting gasometer out of interpreter

* Choosing right gas calculations implementation based on supplied gas

* Moving verification out of gasometer

* MemGasCost benchmark.

Conflicts:
	ethcore/src/evm/benches/mod.rs

* Some simple benchmarks

* Benchmark for simple loop

* Calculating gas_for_memory only when it's actually needed

* Removing superfluous newline [ci skip]

ethcore/src/evm/instructions.rs

@@ -79,7 +79,7 @@ fn test_get_log_topics() {
 	assert_eq!(get_log_topics(LOG4), 4);
 }
 
-#[derive(PartialEq)]
+#[derive(PartialEq, Clone, Copy)]
 pub enum GasPriceTier {
 	/// 0 Zero
 	Zero,